1. Field of the Invention
The present invention relates generally to a mobile communication system that supports multimedia service applications including voice and data, and in particular, to a method and device for transmitting/receiving reverse data rate information.
2. Description of the Related Art
A typical mobile communication system, for example, a CDMA (Code Division Multiple Access) system based on IS-95 supports only voice service. Along with an increasing user demand for data communication and the development of mobile communication technology, the mobile communication system has been evolved to support data service. HDR (High Data Rate), for example, was proposed to support only high-rate data service.
The existing mobile communication systems have been deployed such that voice service and data service are considered separately. Due to an ever increasing demand for contemporaneous delivery of voice and data, the mobile communication technology has reached a position in which we should explore a mobile communication system capable of supporting voice and data services simultaneously. To meet this demand, a so-called 1xEV-DV (Evolution Data and Voice) system has been proposed recently.
The 1xEV-DV mobile communication system enables data transmission on both directions, forward and reverse. In this system, a mobile station transmits reverse data on an SCH (Supplemental channel) to a base station, along with information about the data rate of the SCH on an R-RICH (Reverse Rate Indicator Channel).
FIG. 1 is a block diagram of a conventional reverse data rate information transmitter. The transmitter transmits an R-RICH sequence representing the data rate of a reverse SCH from a mobile station to a base station in the 1xEV-DV mobile communication system, for example.
Referring to FIG. 1, a total of four or seven bits are assigned to an R-RICH sequence to indicate a reverse data rate. Information about the number of SCHs to be used for the mobile station and a set of data rates for each SCH is fed to a controller 101. Then, the controller 101 determines a mapping relation between R-RICH sequence values and the reverse SCH data rates. If up to two SCHs are used, two mapping relations can be set for the case of using one SCH and for the case of using two SCHs. After the mapping, the controller 101 outputs a 4-bit R-RICH sequence in the case of the one SCH, and a 7-bit R-RICH sequence in the case of two SCHs. The code rate of an encoder 102 varies depending on the number of bits assigned to the R-RICH sequence. Code rates of 4/24 and 7/24 are applied respectively to the four-bit R-RICH sequence and the seven-bit R-RICH sequence. In other words, the encoder 102 outputs 24 code symbols regardless of the number of bits of the R-RICH sequence. A sequence repeater 103 repeats the 24-code symbol sequence 15 times (i.e., 16 code symbol sequences occur). A signal point mapper 104 converts the 0s and 1s of 384 code symbols (24 code symbols×16) to +1s and −1s, respectively. A Walsh spreader 105 spreads the 384 code symbols received from the signal point mapper 104 with a predetermined Walsh code of length 64 assigned to the R-RICH. The spread signal is transmitted to the base station in a 20-ms reverse frame.
Table 1 below shows conventional mapping between data rates and R-RICH sequence values when one SCH is used for reverse data transmission. The data rates are listed in an ascending order and 4-bit R-RICH sequence values ranging from 0000 to 1000 are sequentially assigned to the data rates.
TABLE 1data rate (kbps)R-RICH sequence000009.6000119.2001038.4001176.80100153.60101307.20110614.4011110241000
FIG. 2 is a diagram illustrating a signal flow between a mobile station and a base station in a typical call setup procedure.
Referring to FIG. 2, a call setup starts with power-on in the mobile station in step 201. The mobile station notifies a wireless network of its presence by mobile station registration and transmits signaling information to the base station in step 202. The base station determines call setup parameters needed for the mobile station to operate over the wireless network in step 203. The call setup parameters include the number of SCHs to be used for the mobile station and the maximum data rate of each SCH, which are determined considering the type and characteristics of the mobile station, the type of a service to be provided, and the condition of the wireless network. The base station transmits the call setup parameters to the mobile station in step 204. Upon receipt of the call setup parameters in step 205, the mobile station starts to transmit traffic data.
The conventional R-RICH transmitter shown in FIG. 1 considers only the number of SCHs to be used for the mobile station, excluding the maximum data rate of each SCH, in determining the number of bits to be assigned to an R-RICH sequence and the mapping relation between R-RICH sequences and data rates (or data rate combinations). For example, if one SCH is to be used for reverse data transmission, mapping between a particular data rate and an R-RICH sequence is carried out regardless of the maximum data rate of the SCH, as shown in Table 1. The maximum data rate of each SCH can be transmitted by a signaling message at a call setup as shown in FIG. 2, or it can be reset by the signaling message after a call is connected.
A drawback of the conventional R-RICH sequence bit number determining method is that a predetermined number of bits are assigned to the R-RICH sequence even if the number of available data rates is limited by the maximum data rate of each SCH. For example, if one SCH is used and its maximum data rate is 38.4 kbps in Table 1, the mobile station transmits data at or below 38.4 kbps. Therefore, the R-RICH sequence is one of 0000, 0001, 0010, and 0011. Though the four available data rates can be represented in two bits, an extra two bits are additionally assigned. This implies that the encoder 102 encodes data at a code rate 4/24 though a code rate of 2/24 is enough.